1. Field of the Invention
The present invention relates to a resin-molded type semiconductor device wherein a semiconductor chip on which a semiconductor circuit having a great number of circuit elements is formed is mounted on a bed of a lead frame constituted by an electrically conductive plate member, and the bed portions and semiconductor chip are molded and sealed with synthetic resin.
2. Description of the Related Art
The resin-molded type semiconductor device has, for example, an SOJ (Small Outline J lead) package type structure, as shown in FIG. 7A. In this apparatus, a semiconductor chip 52 is mounted on a bed 51 of a lead frame constituted by a metallic plate of, e.g. copper or conductive alloy. The bottom surface of the semiconductor chip 52 is adhered to the bed 51 by use of an adhesive 53 of an epoxy resin, etc. Electrodes 521, 522 . . . of the semiconductor chip 52 attached to the bed 51 are connected to lead terminals 551, 552, . . . , which are led to the outside, by means of bonding wires 541, 542, . . . . The semiconductor chip 52 formed integral with the bed 51, lead terminals 551, 552, . . . and bonding wires 541, 542, . . . are molded with molding resin 56.
In the resin-molded type semiconductor device having the above structure, the molding resin 56 has a moisture-absorbing property, and, as shown in FIG. 7B, moisture 57 accumulates at the interfaces of the bed 51 and molding resin 56. Consequently, for example, when the semiconductor device suffers heat shock in a solder bath in the case of mounting the semiconductor device, the moisture 57 at the interface vaporizes and expands, thereby stretching out the molding resin 56, deforming the outer shape of the molding resin 56 and causing cracks 58 to occur.
The possibility of cracks 58 occurring depends greatly on the kind of material used for mold resin 56, of which a semiconductor device package is formed, the characteristic of the material, the size of the semiconductor chip 52, etc. In particular, as the ratio of the volume of semiconductor chip 52 to the volume of the package increases, the possibility of cracking also increases. Where the crack 58 occurs, bonding wires may be severed, and contaminants and moisture may enter through the crack 58, resulting in corrosion of the semiconductor chip 52.
In addition, in the above device, the bottom surface of the bed 51 is flat, and mold resin 56 is in contact with the flat bottom surface. If crack 58 occurs, the bottom surface of the bed 51 is separated from the mold resin 56, and the reliability of the semiconductor device is lowered.